Data networks convergence is a facet of current and future network infrastructure deployments. The general idea is to converge disparate networks onto a single common network that can support voice, video and other data traffic. Of significance is the introduction of real-time traffic types, especially voice and video traffic, where it is important that the traffic reaches its destination within a predefined time otherwise the user experiences, say, a telephone call or video with delays and jitter, resulting in poor quality. In general these traffic types expect to have a quality quotient applied to the networks they traverse which allows them to have priority over less important traffic types thus ensuring delays and jitter do not occur. This ensures the user's Quality of Service (QoS).
There are a number of elements that can be employed to ensure a network has an acceptable QOS. These include (especially in Internet Protocol (IP) based networks): classification of traffic to determine its type; marking with a priority indicator based on how important the traffic type is; congestion management techniques that ensure high priority traffic is passed first; congestion avoidance schemes that randomly drop packets in the hope of avoiding congestion; traffic conditioning that applies certain bandwidth limits to traffic types; compression to improve bandwidth efficiency; and packet fragmentation to reduce excessive delay because of large packets.
Of importance is the ability for a network to advise that it is about to experience congestion. This is often referred to as incipient congestion and allows a sender to inform a receiver that it is experiencing delay at some point on a particular traffic flow, in response to which the sender can then reduce its transmission rate until the threat of congestion has passed.
An industry standard that supports incipient congestion in data networks that support IP is called Explicit Congestion Notification (ECN) and is defined in RFC 3168—The Addition of ECN to IP.
ECN is a congestion avoidance scheme that marks IP packets in the network instead of dropping them when congestion thresholds are met. Receivers of marked packets can then decrease their transmit rate to avoid the risk of heavy congestion.
As an example and with reference to FIG. 1, Cisco implements a variant of congestion notification which marks two bits in IP packet headers (in the former ToS byte, now redefined by DiffServ to the DSCP field). The two IP bits carry information about IP packet flow between hosts through the routed network and importantly whether congestion has been experienced, this happens when the CE bits are both set.
Cisco Congestion Notification is an extension of Weighted Random Early Detection (WRED) functionality. WRED is an active queue management congestion avoidance mechanism that drops packets as a congestion indicator to end points. This is based on drop thresholds set. When Congestion Notification is configured instead and congestion thresholds are set, instead of dropping packets end hosts receive a signal that allows them to slow down the rate at which they are transmitting. This is illustrated in FIG. 2.
Because ECN is a notification mechanism it must be introduced into other technologies, standards and protocols to effect change. In the case of EP transport, Transmission Control Protocol has a standards definition for this and is ECN compliant. Any endpoint senders and receivers that can successfully negotiate IP ECN and are ECN TCP capable and on an ECN architected network will be able to rate limit traffic using the TCP rate adaptation mechanisms. However, with the convergence of data networks it is necessary to accommodate a plurality of protocols that are not necessarily compliant with ECN. In particular, real time data transmissions often employ a connectionless protocol such as User Datagram Protocol (UDP) which is unsuited to conventional ECN schemes.
It would be desirable to mitigate this problem.